288 UP THE NORTH KIVEK. 



bordering the river, covei'ed with verdure and trees, and 

 with a tortuous channel through the centre of them kept 

 open for the passage of the water. 



The process is a very curious one, and has been observed, 

 and the different steps of the progress of it in particular 

 instances have been carefully noted and recorded by men 

 of science. 



The philosophy of the operation is this : All rivers in 

 their flow bring down with them a great deal of sediment- 

 ary matter, which results in part from the disintegration 

 of the rocks and mountains among which their several 

 branches take their rise, and also from dust blown into 

 them by the wind, and from decayed animal and vegetable 

 substances brought into them by the rains. 



The heavier portions of these substances sink rapidly, 

 and are rolled along the bottoms of the rivers in the form 

 of pebbles and sand. Those that are not so heavy sink 

 more slowly, and where the flow of the stream is rapid and 

 turbulent, their complete subsidence is entirely prevented 

 by the surging and whirl of the water; and in general, 

 the tendency to subsidence on the part of the solid matter 

 held in suspension is determined in a great measure by the 

 slowness or swiftness of the current. 



Now, in all those places where the river is very broad 

 and deep, the motion of the water is very slow, on account 

 of the space through which it moves being so vast, and the 

 quantity moving being so great, that the whole amount 

 that has to pass through during a given time can be trans- 

 mitted by a very slow motion. 



Of course, in all those places where the space is so wide 

 and deep as to form a lake, the deposition takes place much 

 more rapidly than in other places ; and, unless something 

 interferes with the process, the lake, after a certain time, 

 becomes entirely filled up. 



